1. Field of the Invention
The present invention relates to circuitry for a flash unit, and in particular, to an easy-to-control charging circuit for the capacitor of a camera flash unit. The charging circuit includes a modulator for modulating the charging current supplied to the capacitor according to the voltage state of a power supply of the camera, in order to reduce a load on the micro control unit (MCU) and also to avoid fast decreases in battery voltage during charging, thereby prolonging the useful life of the flash unit power supply and of the flash unit itself.
2. Description of the Prior Art
Devices such as a traditional camera, digital camera or cellular phone with camera are regularly equipped with a flash unit in order to cope with the underexposure problem. The flash unit is usually powered by a battery of the camera, or an alternating current (AC), with the relatively large voltage necessary to achieve a xe2x80x9cflashxe2x80x9d being supplied by a flash unit capacitor.
FIG. 1 shows a known flash unit charging circuit 2, including a flash unit capacitor 21, induction coil 22, capacitor 23, transformer 24, diode 25, resistances 26axcx9c26d, transistors 27a, and field effect transistors 27b, and in which one end of the charging circuit 2 is connected to a battery 1 and another end is connected to a control pin 4 arranged to receive a control signal sent by a micro control unit/MCU (not shown). When the control pin 4 receives the control signal, the charging circuit 2 will allow the charging current Ic (average current) to charge the flash unit capacitor 21. However, because the control signal must be sustained during the charging period, the MCU suffers a relatively great load.
In order to shorten the time necessary to charge the flash unit capacitor 21, the charging current Ic is always kept at a relatively high level that leads to a rapid decrease in the voltage of the battery 1. Therefore, the MCU will detect a state of xe2x80x9cbattery lowxe2x80x9d so as to shut off the function of the flash unit. Even though the battery is still useable, the detection of a xe2x80x9cbattery lowxe2x80x9d condition will force a user to replace the battery promptly.
It is an object of the present invention to provide a flash unit charging circuit that eliminates the burden on the MCU of the flash unit.
It is another object of the present invention to provide a control circuit that modulates the charging current so as to extend the life span of the battery.
In order to accomplish these objects, the present invention provides a control circuit including a flash unit capacitor, a voltage converter, and a square-wave oscillator that eliminates the need for continuous control of the charging cycle by the MCU. Furthermore, the voltage converter includes a current control circuit arranged such that, when the voltage of the battery is relatively low, the control circuit will regulate the charging current to reduce the rate of decrease in the voltage of the battery. Therefore, the lifespan of the battery 1 is efficiently extended, albeit at the cost of lengthening the time necessary to charge the flash unit capacitor.